Method for producing carbon nanofiber composite and carbon nanofiber composite

ABSTRACT

A method for a carbon nanofiber composite, which can obtain a carbon nanofiber composite with high productivity and high activity, and which does not require removal of fluidizing materials or dispersing materials, provides a carbon nanofiber composite having improved dispersibility. The method for producing the carbon nanofiber composite includes bringing at least one catalyst and at least one particulate carbon material into contact with at least one gas containing at least one gaseous carbon-containing compound while mechanically stirring the catalyst and the particulate carbon material in a reactor. The carbon nanofiber composite includes carbon nanofibers and at least one particulate carbon material, wherein the particulate carbon material has 70% by volume or more of particles with a particle diameter of 1 μm or less, and/or a median diameter D50 by volume of 1 μm or less.

TECHNICAL FIELD

The present invention relates to a method for producing a carbonnanofiber composite. The present invention also relates to a carbonnanofiber composite.

BACKGROUND ART

Acetylene black, carbon nanofibers (hereinafter referred to as CNFs) andmixtures thereof, which are conductive carbon materials, are used asfillers for imparting conductivity to resins or asconductivity-imparting materials for various batteries, particularlyelectrodes of lithium ion batteries. In particular, the CNFs areattracting attention, because the CNFs are characterized by providinghigher electrical conductivity regardless of a relatively low content ofthe conductive carbon material when the CNFs are used or added. Here,the CNFs generally have an outer diameter of from 5 to 100 nm and anaspect ratio showing a ratio of fiber length to outer diameter of 10 ormore.

An electrode discharge method, a catalytic vapor growth method, a lasermethod, and the like are conventionally used for producing the CNFs.Among them, the catalytic vapor growth method would be most suitable asan industrial production method. In the catalytic vapor growth method,the CNFs are grown from catalyst particles at an elevated temperature ofgenerally 700 to 900° C. by bringing transition metal particles as acatalyst into contact with a raw material gas that is a carbon sourcesuch as acetylene and benzene. This method is attracting attention as amethod of providing CNFs having high purity and high quality at arelatively low temperature.

The catalytic vapor growth method cause a problem that when it iscarried out using a solid catalyst in a fixed bed reactor, then it willbe difficult to obtain uniform CNFs due to non-uniform contact of thecatalyst with the gas or difficulty in equalizing the temperature, sothat entanglement of the resulting CNFs progresses to decreasedispersibility. To solve this problem, prior arts proposes that afluidized bed reactor is used (Patent Documents 1 and 2).

Prior arts also proposes a method for producing CNFs by bringing acarbon-containing gas, a catalyst and previously produced CNFs, duringreaction, into contact with one another in a mechanically agitatingreactor, and illustrates reactors such as a reactor including stirringblades, a rotary reactor and a rotary kiln type reactor (PatentDocuments 3 and 4). In this case, it is also difficult to mechanicallyand uniformly fluidize the catalyst. For example, a prior art proposesthat a fluidizing material such as an inorganic oxide is used (PatentDocument 5).

CITATION LIST

-   Patent Document 1: Japanese Patent No. 3369996 B-   Patent Document 2: Japanese Patent Application Publication No.    2010-030887 A-   Patent Document 3: Japanese Patent Application Publication No.    2006-045051 A-   Patent Document 4: Japanese Patent Application Publication No.    2008-056523 A-   Patent Document 5: Japanese Patent Application Publication No.    2011-213518 A

SUMMARY OF INVENTION Technical Problem

When carrying out the reaction in the fluidized bed, the catalyst andthe produced CNFs have very different fluidities by the gas so that asignificant difference is created between fluidity of the catalyst aloneand the fluidity of the mixture of CNFs and the catalyst when the CNFsare produced, whereby it will be difficult to steadily maintain thecatalyst and the produced CNFs in the reactor, or it will be necessaryto recover and recycle the CNFs/catalyst discharged from the reactoraccompanying with the gas flow, or it will be necessary to provideimprovement for imparting appropriate fluidity to the catalyst. That is,there is a problem that the process window becomes narrower in terms ofselection of catalysts and conditions for stably fluidizing the catalystand the CNFs. The catalytic vapor growth method may also be modified tosupply a gaseous catalyst precursor instead of the solid catalyst toform a catalytic metal species in the reactor, but it is concerned thatthe catalytic metal species is deposited on the previously producedCNFs, and the growing of CNFs from this state results in branched CNFs,thereby decreasing dispersibility. Also, when the fluidizing materialsare used, these fluidizing materials must be separated from the CNFsafter the reaction.

On the other hand, when the CNFs are used as a filler for impartingconductivity to a matrix such as a resin or when they are used as aconductivity imparting agent for a lithium ion battery, thedispersibility of the CNFs are important. However, fine carbon fiberssuch as conventional carbon nanofibers are complicatedly twistedtogether to form a secondary structure and have poor dispersibility.Further, when the CNFs are used alone, the expensive CNFs are oftendiluted with other carbon materials such as carbon black, but efficientdilution is also difficult due to the poor dispersibility of the CNFs.Therefore, it is considered that when the CNFs are used, they aremechanically dispersed beforehand in a jet mill, a ball mill, anultrasonic homogenizer or the like, or they are chemically dispersed byoxidization with nitric acid or the like, but its cost is higher thanthe price of the CNFs, which may be economically disadvantageous.Further, there is also a problem that these strong dispersing treatmentscause breakage or oxidation of the CNFs, whereby the important electricconductivity may be deteriorated.

In the future, various conductive materials will be required dependingon the applications. Then, a composite made of heterogeneous carbonmaterials would provide various conductive properties depending on itsshape and proportion. Particularly, CNFs with a larger aspect ratio (aratio of length to diameter) has improved conductivity impartingperformance, and a combination the CNFs and a conventional inexpensivecarbon material such as carbon black would be able to provide apossibility that the combination provides a conductive material havingimproved cost performance. However, since the conventional CNFs havepoor dispersibility as described above, it is difficult to uniformly mixthe conventional CNFs with other carbon materials by simple mixing.Therefore, there would be a need for a method of efficiently producing ahighly dispersible composite made of CNFs and other carbon materials.

In view of the above problems and actual circumstances, an object of thepresent invention is to provide a method for a carbon nanofibercomposite, which can obtain a carbon nanofiber composite with highproductivity and high activity, and which does not require removal offluidizing materials or dispersing materials. Another object of thepresent invention is to provide a carbon nanofiber composite havingimproved dispersibility.

Solution to Problem

In one aspect, the present invention provides a method for producing acarbon nanofiber composite, comprising bringing at least one catalystand at least one particulate carbon material into contact with at leastone gas containing at least one gaseous carbon-containing compound whilemechanically stirring the at least one catalyst and the at least oneparticulate carbon material in a reactor.

In another aspect, the present invention provides a carbon nanofibercomposite comprising carbon nanofibers and at least one particulatecarbon material, wherein the carbon nanofiber composite comprises 70% byvolume of particles with a particle diameter of 1 μm or less and/or amedian diameter D50 by volume of 1 μm or less.

Advantageous Effects of Invention

According to the present invention, there is provided a method forproducing a carbon nanofiber composite including carbon nanofibers andat least one particulate carbon material with high productivity andactivity. Since the carbon nanofiber composite according to the presentinvention can exhibit high dispersibility and high conductivity, thecarbon nanofiber composite improves the conductive network and thus canbe suitably used as a conductive filler or a conductivity impartingmaterial.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic view of a horizontal rotary reactor used inExamples.

FIG. 2 is an end view taken along the line A-A′ of FIG. 1.

FIG. 3 shows a TEM photograph of a CNF portion in the CNF compositeobtained in Example 1.

FIG. 4 shows a TEM photograph of a CNF portion in the CNF compositeobtained in Example 8.

FIG. 5 shows a TEM photograph of a CNF portion in the CNF compositeobtained in Example 9.

FIG. 6 shows a result of dispersibility evaluation (a graph showing afrequency distribution of % by volume versus particle diameter) of a CNFcomposite obtained in Example 1.

FIG. 7 shows a result of dispersibility evaluation (a graph showing afrequency distribution of % by volume versus particle diameter) of a CNFcomposite obtained in Comparative Example 1.

FIG. 8 shows an example of a SEM photograph of a CNF composite obtainedin Example 2.

DESCRIPTION OF EMBODIMENTS Definition of Terms

The term “carbon nanofiber” (CNF) as used herein refers to a carbonnanofiber having an average outer diameter of from 5 to 100 nm, anaspect ratio showing a ratio of a fiber length to an average outerdiameter of 10 or more, also including a multi-walled carbon nanotube(MWCNT), and more preferably a carbon nanofiber mainly based on themulti-walled carbon nanotube. Here, the fiber length is a length of CNFto be measured. Even if the CNF is bent, the fiber length is determinedby measuring the length of the tube along its shape. The lengths ofarbitrary 100 or more CNFs are measured from a photograph obtained byTEM (transmission electron microscope) or SEM (scanning electronmicroscope), and a number average length for the measured lengths isdetermined. The outer diameter refers to a length of a line segment thatcan penetrate the CNF to be measured, among line segments in thedirection perpendicular to the direction of the fiber length. In thepresent invention, the average outer diameter of CNFs can be determined,for example, by measuring the outer diameters at arbitrary 30 positionsfrom a photograph obtained by TEM for one CNF to be measured, andcalculating an average value for the measured outer diameters.

The definition of the carbon nanofiber (CNF) as used herein includes asingle-walled carbon nanotube (SWCNT) and a multi-walled carbon nanotube(MWCNT). Also included are a non-hollow cup-stacked nanofiber and aplatelet nanofiber. Although the single-walled carbon nanotube ischaracterized by exhibiting high conductivity, the nanotube haspractical problems that it has isomers due to chirality and has a bundlestructure. The cup-stacked nanofiber and platelet nanofiber have lowerconductivity than that of the carbon nanotube. Therefore, themulti-walled carbon nanotube (MWCNT) is most preferable as the carbonnanofiber (CNF) in the present embodiments. FIG. 3 shows a TEMphotograph of the CNF synthesized in Example 1 as a representativeexample of the carbon nanofibers according to the present invention. Itshows that the carbon nanofiber is the multi-walled carbon nanotube.

The term “synthesis activity” as used herein refers to a mass of theCNFs obtained per unit time per unit active species mass. The catalyticactivity refers to a mass of the CNFs obtained per unit time per unitcatalyst mass. The productivity refers to a mass of the CNFs obtainedper unit catalyst mass.

According to the present invention, the following carbon nanofibercomposites and methods for producing the same are illustrativelyprovided:

(1) A method for producing a carbon nanofiber composite, comprisingbringing at least one catalyst and at least one particulate carbonmaterial into contact with at least one gas containing at least onegaseous carbon-containing compound while mechanically stirring thecatalyst and the particulate carbon material in a reactor.(2) The method for producing the carbon nanofiber composite according tothe aspect (1), wherein the reactor comprises a rotary reactor.(3) The method for producing the carbon nanofiber composite according tothe aspect (1) or (2), wherein the particulate carbon material comprises70% by volume or more of particles with a particle diameter of 1 μm orless, and/or a median diameter D50 by volume of 1 μm or less.(4) The method for producing the carbon nanofiber composite according toany one of the aspects (1) to (3), wherein the particulate carbonmaterial comprises one or more selected from graphite, carbon black, andgraphene.(5) The method for producing the carbon nanofiber composite according toany one of the aspects (1) to (4), wherein the gaseous carbon-containingcompound comprises carbon monoxide.(6) The method for producing the carbon nanofiber composite according toany one of the aspects (1) to (5), wherein the method comprises carryingout the reaction at a temperature ranging from 550° C. to 900° C. in thereactor.(7) The method for producing the carbon nanofiber composite according toany one of the aspects (1) to (6), wherein the catalyst comprises one ormore of the following (A), (B) and (C):(A) a catalyst in which an active species mainly based on cobalt issupported on a support comprising an oxide containing magnesium;(B) a catalyst in which an active species mainly based on cobalt issupported on a support comprising an oxide containing titanium; and(C) a catalyst in which an active species mainly based on any of ironand nickel is supported on a carbon particle support.(8) The method according to any one of the aspects (1) to (7), whereinthe method comprises allowing the gas containing the gaseouscarbon-containing compound to flow such that the gaseouscarbon-containing compound flows through the reactor at a flow rate of0.1 NL/g-active species⋅minute or more.(9) The method for producing the carbon nanofiber composite according toany one of the aspects (1) to (8), wherein the gas containing thegaseous carbon-containing compound further comprises hydrogen.(10) A carbon nanofiber composite comprising carbon nanofibers and atleast one particulate carbon material, wherein the particulate carbonmaterial comprises 70% by volume or more of particles with a particlediameter of 1 μm or less, and/or a median diameter D50 by volume of 1 μmor less.(11) The carbon nanofiber composite according to the aspect (10),wherein the carbon nanofiber composite comprises from 10 to 90% by massof the carbon nanofibers.(12) The carbon nanofiber composite according to the aspect (10) or(11), wherein the particulate carbon material comprises carbon black.(13) A method for removing a residual catalyst, comprising subjectingthe carbon nanofiber composite according to any one of the aspect (10)to (12) to a heat treatment in an inert gas atmosphere at a temperatureof 1500° C. or higher and 2500° C. or lower.(14) A conductive resin composition comprising the carbon nanofibercomposite according to any one of the aspects (10) to (12).(15) A dispersion, an ink or a paint comprising the carbon nanofibercomposite according to any one of the aspects (10) to (12).

Reactor

The reactor that can be used in the present invention is notparticularly limited as long as it is a reactor which has any shapecapable of accommodating the catalyst and the particulate carbonmaterial in a gas atmosphere containing the carbon-containing compound,and which has a function of mechanically stirring the catalyst and theparticulate carbon material by mechanical operation of a part or thewhole of the reactor. A movable part(s) of the reactor may be a stirringblade(s), a paddle(s) or the like, or the reactor itself may rotate orvibrate. An example of the latter may be a rotary kiln reactor. Theconcept of “a reactor which has a function of mechanically stirring” asused herein does not include any concept of a fluidized bed reactor inwhich a catalyst or the like is dispersed by a fluid such as a gas. Inaddition to the above problems, the use of the fluidized bed reactorresults in easy separation of the particulate carbon material and theCNFs due to the difference in fluidity between the particulate carbonmaterial and the CNFs, so that it may be difficult to obtain a carbonnanofiber composite having a desired composition. In the presentinvention, the reactor which has a function of mechanically stirring maypreferably be a rotary reactor, and more preferably a horizontal rotaryreactor which may have a slight gradient, such as the rotary kilnreactor. The catalyst and the granular carbon material in the reactorare mechanically stirred, whereby they can be brought into contact withthe carbon-containing gas as a raw material, with high uniformity.Further, the particulate carbon material serves to inhibit aggregationof the produced CNFs, because the particulate carbon material is stirredtogether with the produced CNFs. The reaction in this reactor may bebatch type or continuous type.

Gas Containing Gaseous Carbon-Containing Compound

In the present invention, the gaseous carbon-containing compound in thegas containing the gaseous carbon-containing compound plays a role as amaterial for reaction and can be converted to CNFs by catalysis.Examples of the carbon-containing compound that can be used include, butnot limited to, a hydrocarbon having from 1 to 10 carbon atoms, carbonmonoxide, carbon dioxide and mixtures thereof. Examples of thehydrocarbon include methane, ethane, propane, ethylene, propylene,benzene, toluene and acetylene. In the present invention, carbonmonoxide is most preferably used. By using carbon monoxide, CNFs havinghigh crystallinity and high conductivity can be produced even at arelatively low reaction temperature.

The gas containing the gaseous carbon-containing compound may containother components, in particular, components for modifying the propertiesof CNFs and components for improving productivity during production. Forexample, it may contain an inert gas such as nitrogen and argon, or itmay contain hydrogen capable of suppressing deactivation of the catalystand improving productivity. The molar fraction of hydrogen in the gascontaining the gaseous carbon-containing compound may preferably be 0.1%or more, and more preferably 1% or more, and still more preferably 3% ormore. Further, the total molar fraction of the gaseous carbon-containingcompound in the gas containing the gaseous carbon-containing compoundmay preferably be 50% or more, in terms of providing higher synthesisactivity.

Particulate Carbon Material

The particulate carbon material used in the present invention can serveas a fluidizing dispersant for increasing the catalytic activity andproductivity during the production of CNFs and improving thedispersibility of CNFs. Various carbon materials can be used accordingto the purposes. In the particulate carbon material, particles eachhaving a particle diameter of 1 μm or less may preferably account for70% by volume or more, and more preferably 80% by volume or more, andeven more preferably 90% by volume or more, in terms of improving thedispersibility of CNFs and CNF composite. Further, in the particulatecarbon material, a median diameter D50 by volume may preferably be 1 μmor less, and more preferably 0.5 μm or less, in terms of improving thedispersibility of CNFs.

The dispersibility may be evaluated by any known method. Preferably, inthe present invention, the fraction of volume occupied by the particleseach having a particle diameter of 1 μm or less in the particulatecarbon material and the median diameter D50 by volume of the particulatecarbon material can be adopted as the dispersibility evaluation method.These are measured by the same method as that of a dispersibilityevaluation method of the CNF composite as described above.

Standardized Pretreatment for Measuremen]

6 mL of an aqueous solution of 0.1% by mass of sodium carboxymethylcellulose (CMCNa) is prepared, and a dispersion solution is preparedsuch that a concentration of the particulate carbon material in theaqueous solution is 0.1% by mass. Using an ultrasonic homogenizer (forexample, Smurt NR 50-M from Microtec Co., Ltd.;⋅frequency: 20 kHz;output: 50 W), the dispersion solution was subjected to ultrasonicirradiation for 40 seconds under conditions of an auto power mode andoutput of 50%, suspended and homogenized to prepare a sample solution.

Particle Size Distribution Measuremen]

The sample solution is then subjected to particle size distributionmeasurement according to the laser diffraction/scattering method (ISO13320: 2009).

As with measurement of the CNF composite as described below, the samples(the particulate carbon materials or comparative samples) used in thismeasurement are not subjected to any dispersion treatment other than theabove standardized pretreatment for measurement.

The particulate carbon materials satisfying the above conditions can besuitably used in the present invention, and specific preferable examplesinclude carbon black, graphene and graphite. Examples of carbon blackinclude acetylene black, furnace black, and ketjen black. The acetyleneblack is available from Denki Company Limited, Ketjen black is availablefrom Lion Corporation, and the furnace black is available from TIMCALltd., Mitsubishi Chemical Corporation, or Tokai Carbon Co., Ltd. Thesecarbon blacks may be denatured, modified or doped with boron, nitrogenor the like. As the graphene, a known substance may be used. Theexamples of the graphite include natural graphite or artificialgraphite, which is fine graphite satisfying the above the mediandiameter condition. Examples of natural graphite include massivegraphite, scaly graphite and earthy graphite, but graphite particlesderived from these may be used. Natural graphite purified to improve thepurity or flaked graphite may also be preferably used. Particulategraphite derived from the natural graphite is for example available fromNippon Graphite. Examples of the artificial graphite particles includethose made from coke, pitch, coal tar, or resins. This category alsoincludes mesophase carbon and glassy carbon. The particulate graphitederived from the artificial graphite is available from, for example,Tokai Carbon Co., Ltd., Hitachi Chemical Co., Ltd., Showa Denko, K.K, orNippon Graphite. These graphite particles can be suitably used even forthose used as negative electrode materials of lithium ion batteries. Thecarbon black is most preferably used in the present invention. When thecarbon black is used, the CNF composite having very improveddispersibility can be obtained due to good dispersibility of carbonblack used.

The concept of the particulate carbon material in the present inventiondoes not include fibrous carbon such as carbon fibers, activated carbonfibers or carbon nanofibers (CNFs). When these fibrous carbons are usedas fluidizing dispersants, the intervening fibrous carbon is entangledin CNFs grown by the catalytic reaction, so that the dispersibility ofthe resulting CNF composite may be decreased. More particularly, in thepresent invention, the particulate carbon material may preferably have aratio (aspect ratio) of a long diameter to a short diameter in a rangeof from 1 to less than 10. Here, the long diameter refers to a length ofthe longest line segment that can penetrate the carbon material to bemeasured, and the short diameter refers to a length of the longest linesegment that can penetrate the carbon material to be measured, amongline segments in the direction perpendicular to the long diameter. Theseparticulate carbon materials may form a structure in which primaryparticles of the particulate carbon material are in contact with andbonded to each other.

Catalyst

In the producing method according to the present invention, a knowncatalyst for CNF synthesis may be used. Even if a catalyst for CNFsynthesis that cannot obtain CNFs with good dispersibility in theconventional synthesis in the fixed bed or synthesis under mechanicalstirring is used, the catalyst can produce a CNG composite with moreimproved dispersibility when used in the method according the presentinvention.

It is preferable to employ, among the catalysts for CNF synthesis, oneor more catalysts selected from the following catalysts (A) to (C) thatcan produce relatively high dispersible CNFs even in the conventionalproducing method. When synthesizing CNFs by the catalyst alone, theproduction conditions for the CNFs will be limited even if the catalystcan provide highly dispersible CNFs, and the activity is also relativelylow. For example, the conditions under which highly dispersible CNFs areobtained may be those which sacrifice the original high CNF productivityper catalyst. According to the method of the present invention, it ispossible to efficiently produce the CNF composite having improveddispersibility under higher CNF productivity conditions and to widelyexpand the process window therefor.

Catalyst A

Catalyst A is a catalyst for producing carbon nanofibers, in which anactive species mainly based on cobalt is supported on a support composedof an oxide containing magnesium (the oxide is a concept including acomplex oxide) (hereinafter, the catalyst is referred to as“cobalt-magnesium oxide supported catalyst”).

Active Species of Catalyst A

The cobalt-magnesium oxide supported catalyst according to the presentinvention is mainly based on cobalt as a substantial active species forCNF production. Cobalt may be in the form of not only metallic cobaltbut also in the form of a compound such as an oxide, hydroxide, hydrousoxide, nitrate salt, acetate salt, oxalate salt and carbonate salt. Theactive species may contain an element(s) of Groups 4 to 12 as acomponent(s) other than cobalt. These include the Iron Group such asiron and nickel; manganese; and molybdenum. However, it is preferablethat at least 60 mol % or more, and preferably 80 mol % or more, andmost preferably 90 mol % or more, of the component of Groups 4 to 12contained as an active species of the catalyst is the cobalt component(in mol % of cobalt element). As other active species, an element(s) ofGroups 1 to 3 or Group 14 may be contained.

Support for Catalyst A

As a support on which the active species is supported, an oxidecontaining magnesium having a specific surface area of from 0.01 to 5m²/g may be preferably used. Examples of the oxide containing magnesiuminclude spinel oxides and perovskite oxides containing magnesium oxideor magnesium, and the like. Among them, magnesium oxide is mostpreferable as the support. The specific surface area of the oxidecontaining magnesium may more preferably be from 0.01 to 4 m²/g, andstill more preferably from 0.03 to 3 m²/g. If the specific surface areais less than 0.01 m²/g, the crystallinity and electrical conductivity ofthe resulting CNFs may be reduced. If the specific surface area exceeds5 m²/g, the synthesis activity and dispersibility of the resulting CNFsmay be deteriorated. The support may contain an oxide(s) of other metalelement(s) selected from Groups 1 to 3 and Group 14. The content of theoxide containing magnesium in the support may preferably be at least 50%by mass or more, and more preferably 90% by mass or more, and mostpreferably 98% by mass or more. If the oxide containing magnesium isless than 50% by mass, the synthesis activity may be decreased.

As the rate of supported cobalt increases, the catalytic activity tendsto increase and the crystallinity of the resulting CNFs tends to beimproved. However, if the rate of supported cobalt is too large, theparticle diameter of cobalt is increased and the synthesis activity maybe decreased. On the other hand, if the rate of supported cobalt issmall, the synthesis activity is increased but the catalytic activitytends to be decreased, and the crystallinity or the conductivity of theresulting CNFs may be decreased. Therefore, the rate of cobalt supportedon the support may arbitrary, but preferably be from 3 to 150% by mass,and most preferably from 5 to 90% by mass. It should be noted that thesupported rate herein, including the cases of catalyst B and catalyst C,is calculated based on the following equation:

supported rate=mass of active species(as metal component)/mass ofsupport×100(%).

When supporting cobalt on the support, the supporting method is notparticularly limited. For example, cobalt can be supported on thesupport by impregnating the support with a non-aqueous solution (forexample, an ethanol solution) or an aqueous solution in which a salt ofcobalt is dissolved, or a mixed solution thereof, sufficientlydispersing and mixing it, and then drying it, and firing it in air at anelevated temperature (for example, from 300 to 600° C.). Alternatively,the support may be simply impregnated with a non-aqueous solution (forexample, ethanol) or an aqueous solution in which a salt of cobalt isdissolved, sufficiently dispersed and mixed, and then subjected tomoisture removal and dried. Alternatively, the support may beimpregnated with a non-aqueous solution or an aqueous solution in whicha salt of cobalt is dissolved, sufficiently dispersed and mixed, andthen neutralized with alkali, subjected to moisture removal, dried andfired. A method such as spray drying may be used for drying.

When using this catalyst, the reaction temperature for synthesizing CNFsis as follows, particularly preferably 600° C. or more and 750° C. orless.

Catalyst B

Catalyst B is a catalyst in which an active species mainly based oncobalt is supported on an oxide containing titanium (the oxide is aconcept including a complex oxide) (hereinafter, the catalyst isreferred to as “cobalt-titanium oxide supported catalyst”).

Active Species of Catalyst B

The cobalt-titanium oxide supported catalyst according to the presentinvention is mainly based on cobalt as a substantial active species forCNF production. The description of the active species of the catalyst Bis the same as that of the active species of the catalyst A, includingthe preferred embodiments.

Support for Catalyst B

The support that can be preferably used includes an oxide containingtitanium and having a specific surface area of from 20 to 140 m²/g.Examples of the oxide containing titanium include spinel oxides andperovskite oxides containing titanium oxide or titanium, and the like.Among them, titanium oxide is most preferable as the support. Thespecific surface area may more preferably be from 25 to 100 m²/g, andstill more preferably from 40 to 70 m²/g. The specific surface area of20 m²/g or more leads to improved synthesis activity. Further, thespecific surface area of 140 m²/g or less provides an advantage thathigh synthesis activity can be obtained. When titanium oxide is used asthe support, the support may be titanium oxide alone or a mixture withother oxides. Titanium oxide may preferably have a rutile crystalstructure or a mixed structure of a rutile structure and an anatasestructure, in view of providing high synthesis activity, but it may havean anatase structure. Examples of the complex oxide containing titaniuminclude potassium titanate, barium titanate, strontium titanate, calciumtitanate, magnesium titanate, lead titanate, aluminum titanate andlithium titanate. Among them, barium titanate may be preferred becauseit can lead to high synthesis activity. When the complex oxidecontaining titanium is used as the support, the support may be thecomplex oxide alone or a mixture with other oxides. The proportion bymass of the oxide containing titanium in the support may preferably beat least 50% by mass, and more preferably 90% by mass or more, and mostpreferably 98% by mass or more. The content of the oxide containingtitanium of 50% by mass or more can allow improvement of conductivity,crystallinity and the like.

The higher the rate of supported cobalt, the higher the yield of CNFswill be. However, if the supported rate is too high, the particle sizeof cobalt will increase and the CNFs produced will become thick, so thatthe synthesis activity per an active species tends to decrease. On theother hand, if the rate of supported cobalt is small, the particle sizeof the supported cobalt will become small and fine carbon nanotubes willbe obtained, but the synthesis activity per a catalyst tends to bedecreased. The optimal rate of supported cobalt will vary depending onthe pore volume, the outer surface area and the supporting method of thesupport, but it may preferably be from 0.1 to 50% by mass, and morepreferably 1 to 40% by mass, and most preferably 5 to 30% by mass. Therate of supported cobalt of 0.1 to 50% by mass improves the synthesisactivity per a catalyst and provides an advantage of cost.

When supporting cobalt on the support, the supporting method is notparticularly limited. For example, the active species can be supportedon the support by impregnating the support with a non-aqueous solution(for example, an ethanol solution) or an aqueous solution in which asalt of cobalt is dissolved, or a mixed solution thereof, sufficientlydispersing and mixing it, and then it to moisture removal and firing itin air (for example, at a temperature of from 300 to 700° C.).Alternatively, the support may be simply impregnated with a non-aqueoussolution (for example, ethanol) or an aqueous solution in which a saltof cobalt is dissolved, sufficiently dispersed and mixed, and thensubjected to moisture removal and dried. Alternatively, the support maybe impregnated with a non-aqueous solution or an aqueous solution inwhich a salt of cobalt is dissolved, sufficiently dispersed and mixed,and then neutralized with alkali, subjected to moisture removal, driedand fired. A method such as spray drying may be used for drying.

When using the catalyst, the reaction temperature for synthesizing CNFsis as follows, particularly preferably 600° C. or more and 750° C. orless.

Catalyst

Catalyst C is a catalyst in which an active species mainly based on anyof iron and nickel is supported on a carbon particle support.

Active Species of Catalyst C

As the active species of the catalyst, an active species mainly based onany of iron and nickel is used. Here, iron and nickel may be in the formof not only a metal but also a compound such as an oxide, hydroxide,hydrous oxide, nitrate salt, acetate salt, oxalate salt and carbonatesalt.

However, preferably 70% by mass or more, and more preferably 95% by massor more, of components excluding the support may be preferably composedof iron and nickel (a total value as metal components of both), from theviewpoint that the active species of the catalyst mainly based on any ofiron and nickel suitably functions. By fulfilling this condition, CNFshaving relatively high crystallinity, high conductivity and highdispersibility can be obtained with higher CNF synthesis activity.

When using the catalyst of the active species mainly based on any ofiron and nickel, the resulting CNFs will be substantially carbonnanotubes (CNTs). On the other hand, when the active species is mainlybased on iron, the resulting CNFs will mainly contain a herringbonestructure. The active species mainly based on any of iron and nickel maybe preferred, because the resulting CNFs preferably have the carbonnanotube (CNT) structure, considering that it provides higherconductivity (lower volume resistivity) and higher crystallinity,particularly in view of the mechanical strength as fibers.

When the active species is mainly based on iron and nickel, the activespecies may have any mass ratio of iron to nickel. When the activespecies is mainly based on any of iron and nickel, the crystallinity anddispersibility of the resulting CNFs are improved. The most preferableratio of iron to nickel in the case of using the catalyst C may be in arange of Fe:Ni=6:4 to 4:6. When the composition of the active species isin this range, the resulting CNFs can exhibit extremely higherdispersibility together with higher conductivity and crystallinity.

Components that may be contained in a small amount, other than iron andnickel, includes cobalt, manganese, molybdenum, copper, and tungsten,and it is not excluded that components of Groups 1 to 3 or Group 14 arealso contained.

Support for Catalyst C

As the support, carbon particles each having a specific surface area offrom generally 0.01 to 200 m²/g, preferably from 0.1 to 20 m²/g, areused. Here, the carbon particles that can be illustrated includegraphite particles, various carbon blacks and the like, preferablygraphite particles, more particularly natural graphite or artificialgraphite. By using such carbon particles as the support, very high CNFsynthesis activity can be obtained and the CNF-carbon material compositecan be efficiently obtained. In contrast to the ordinary metal oxidebased supports, the use of the conductive carbon particles as thesupport results in a CNF-carbon material composite having excellentconductivity without removing the support from the resulting CNFs.

It is preferable that the carbon particle as used herein have a ratio(aspect ratio) of a long diameter to a short diameter in a range of from1 to less than 10. The long diameter refers to a length of the longestline segment that can penetrate the graphite material to be measured,and the short diameter refers to a length of the longest line segmentthat can penetrate the graphite material to be measured, among linesegments in the direction perpendicular to the long diameter. The carbonparticles may form a structure in which the particles are brought intocontact with and bound to each other. Therefore, the concept of thecarbon particles does not include carbon materials having structuresother than the particulate structure, such as carbon fibers, activatedcarbon fibers and carbon nanofibers (CNFs). When the carbon materialhaving a structure other than the particulate structure is used as thesupport, the resulting CNFs and these materials may be entangled witheach other, so that the dispersibility may be lowered.

Surprisingly, the CNF-carbon material composite having excellentdispersibility can be obtained by using graphite particles each having aspecific surface area of, most preferably, from 0.1 to 20 m²/g as thesupport for the above active species. The reason would be that the CNFsdirectly obtained by the CNF synthesis reaction using the catalyst C isessentially highly dispersible and has less interaction with thegraphite particle support, for example, less entanglement. Further, theuse of such a graphite particle support provides an advantage that themetal active species is easily separated from the support due to lowerbonding strength to the metal active species.

On the other hand, if the carbon particles each having a specificsurface area of less than 0.1 m²/g or more than 20 m²/g, for examplecarbon black, are used, high activity may not be obtained. Moreparticularly, if the specific surface area is less than 0.1 m²/g, thesupported metal catalyst is insufficiently dispersed so that theactivity tends to be decreased. On the other hand, if the specificsurface area exceeds 20 m²/g, the synthesis activity of the CNFs isdecreased, so that there is concern that entanglement between theresulting CNFs or the CNFs including the support becomes intense and thedispersibility is lowered.

Examples of preferable natural graphite include massive graphite, scalygraphite and earthy graphite, and graphite particles derived from thesemay also be used. For example, it is preferable to use natural graphiteor flaked graphite purified to improve purity. Particulate graphitederived from natural graphite can be obtained from Nippon Graphite, forexample. The artificial graphite particles that can be suitably usedinclude those which are produced from coke, pitch, coal tar and resinsand which satisfy the above specific surface area. This category alsoincludes mesophase carbon and glassy carbon. The particulate graphitederived from the artificial graphite can be available from, for example,Tokai Carbon Co., Ltd., Hitachi Chemical Co., Ltd., Showa Denko, K.K, orNippon Graphite. These graphite particles can be suitably used,including those used as negative electrode materials of lithium ionbatteries.

When supporting the active species on the support, the supporting methodis not particularly limited and any known method may be used. Forexample, the active species can be supported on the graphite particlesupport by impregnating the graphite particle support with a non-aqueoussolution (for example, an ethanol solution) or an aqueous solution inwhich a metal salt is dissolved, or a mixed solution thereof,sufficiently dispersing and mixing it, and then subjecting it tomoisture removal and drying it. Alternatively, the graphite particlesupport may be impregnated with a non-aqueous solution or an aqueoussolution in which a metal salt is dissolved, sufficiently dispersed andmixed, and then neutralized with alkali, subjected to moisture removaland dried. A method such as spray drying may be used for drying. Othermethods are described, for example, in “Catalyst Preparation Handbook”,supervised by Masakazu Iwamoto, Apr. 25, 2011, issued by the CatalysisSociety of Japan NTS.

The rate of the active species supported on the graphite particlesupport may vary depending on the pore volume of the support, the outersurface area and the supporting method, and may generally be from 1 to50% by mass. If the supported rate is less than 1% by mass, thesynthesis activity tends to be decreased. On the other hand, if thesupported rate exceeds 50% by mass, the CNFs produced will becomethicker or the synthesis activity may be decreased. The supported ratemay most preferably be from 1 to 10% by mass, in terms of the synthesisactivity.

When using the catalyst, the reaction temperature for synthesizing CNFsis as follows, particularly preferably 550° C. or more and 650° C. orless.

By using the catalyst selected from these preferred catalysts (A), (B)and (C) and carrying out the synthesis under suitable reactionconditions for each catalyst, CNFs having high crystallinity, highconductivity and high dispersibility can be obtained. Thesecharacteristics can also be achieved in the CNFs obtained by carryingout the synthesis reaction without using the fluidizing dispersant madeof the granular carbon material. However, the conditions for producingthe CNFs alone having these characteristics are limited, and synthesisactivity is also not sufficient. There is a problem that thedispersibility is significantly deteriorated particularly when one ofthe conditions is not satisfied. There is also a problem that thedispersibility of the CNFs and the synthesis activity or productivity ofthe CNFs are in a trade-off relationship, so that the activity orproductivity are lowered when attempting to improve the dispersibility.The producing method according to the present invention can produce ahighly dispersible CNF composite, in which by using the catalystselected from those catalysts (A) to (C) capable of providing CNFs withhigh crystallinity, high conductivity and high dispersibility, the CNFcomposite having higher conductivity and dispersibility can be producedwith higher activity and productivity.

In addition to the catalysts (A), (B) and (C) suitably used in thepresent invention, catalysts that can be used in the present inventionare known catalysts for CNF synthesis. When using these known catalysts,a gas containing a gaseous carbon-containing compound suitable for eachcatalyst can be used as a raw material. Further, as reaction conditions,those suitable for each of those catalysts can be appropriately selectedfrom the reaction condition ranges as shown below.

Reaction Conditions of CNF

In one embodiment of the method for producing the carbon nanofibercomposite according to the present invention, the method includesbringing the catalyst and the particulate carbon material into contactwith at least one gas containing at least one gaseous carbon-containingcompound while mechanically stirring the catalyst and the particulatecarbon material in a reactor to produce a composite of carbon nanofibersand the particulate carbon material.

The catalyst and the particulate carbon material may be mixed at anymixing ratio in the reactor, depending on the type of the catalyst andthe support, the rate of supported active species and the like.

The total pressure of the gas containing the gaseous carbon-containingcompound may preferably be 0.98 MPa or less, and more preferably 0.5 MPaor less, and still more preferably 0.3 MPa or less. When the totalpressure is more than 0.98 MPa, the costs and utilities of the pressurefacility equipment will be increased for the production. Further, whenthe pressure is greatly reduced as compared with 0.1 MPa (atmosphericpressure), for example, when it is less than 0.08 MPa, it is difficultto seal the high temperature reactor for preventing atmospheric (oxygen)contamination, which may be preferable. Therefore, the total pressure ofthe gas containing the gaseous carbon-containing compound may preferablybe 0.03 MPa or more, and more preferably 0.04 MPa or more.

In addition, it is preferable to allow the gas containing the gaseouscarbon-containing compound to flow at a predetermined flow rate into thereactor in order to improve the synthesis activity. More particularly,it is preferable to allow the gas containing the gaseouscarbon-containing compound to flow into the reactor such that thegaseous carbon-containing compound flows at a flow rate of 0.1NL/g-active species⋅minute or more. The flow rate of the gaseouscarbon-containing compound may more preferably be 1 NL/g-activespecies⋅minute or more. However, if the flow rate of the gaseouscarbon-containing compound is excessively increased, the utility costfor preheating will be increased, and the synthesis activity may beconversely lowered. Therefore, the flow rate of the gaseouscarbon-containing compound may preferably be 200 NL/g-activespecies⋅minute or less, and more preferably 100 NL/g-activespecies⋅minute or less. The “NL” represents an amount L (liter) of thegas converted into the standard state (0° C., 1 atm), and the“NL/g-active species⋅minute” means an amount of the gas per one minutein the presence of a unit active species (per 1 g of active species).

The reaction temperature of the present invention is optional dependingon the catalysts used, and may be generally from 550 to 900° C.,typically from 600 to 700° C. If the reaction temperature is lower than550° C., the synthesis activity may be decreased, or the crystallinity,conductivity and dispersibility of CNFs may be decreased. On the otherhand, if the temperature is higher than 900° C., the synthesis activitymay be decreased.

Production conditions in the case where carbon monoxide that is mostsuitable as a raw material gas is used will be now exemplarilydescribed. Carbon monoxide used as a raw material may be employed as amixed gas with carbon dioxide or hydrogen, or may contain an inert gassuch as nitrogen gas. When the partial pressure (absolute pressure) ofcarbon monoxide gas is less than 0.04 MPa, the synthesis activity may bedecreased or the crystallinity and conductivity of the resulting CNFsmay be decreased. Further, when the partial pressure (absolute pressure)of carbon monoxide gas is higher than 0.98 MPa, the dispersibility ofthe resulting CNFs may be decreased, or the catalyst may be severelydeactivated so that the synthesis activity may be decreased. The partialpressure (absolute pressure) of carbon monoxide may preferably be from0.04 to 0.98 MPa, and more preferably from 0.05 to 0.3 MPa, and mostpreferably from 0.05 to 0.1 MPa.

The partial pressure of the hydrogen gas may preferably be from 1 to100%, and more preferably from 5 to 60%, relative to the partialpressure of carbon monoxide gas. If the partial pressure of the hydrogengas relative to the partial pressure of the carbon monoxide gas is morethan 100%, the synthesis activity may be decreased or the crystallinityand conductivity of the resulting CNFs may be decreased. If the partialpressure of the hydrogen gas is less than 1%, the catalyst may bedeactivated in an early stage and the synthesis activity may bedecreased.

It should be noted that the partial pressure of the hydrogen gasrelative to the partial pressure of the carbon monoxide gas can becalculated by the following equation:

Partial Pressure of Hydrogen Gas relative to Partial Pressure of CarbonMonoxide Gas=Molar Ratio of Hydrogen Gas/Molar Ratio of Carbon MonoxideGas×100(%).

For example, in the case of a mixed gas whose composition of the rawmaterial gas is CO/H₂/N₂=85/15/0 as shown in Table 2, the partialpressure of the hydrogen gas relative to the partial pressure of thecarbon monoxide can be calculated as the partial pressure of thehydrogen gas relative to the partial pressure of the carbon monoxidegas=15/85×100=18(%).

It is preferable that the flow rate of the carbon monoxide gas is 0.1NL/g-active species⋅minute or more while satisfying the aboveconditions. The flow rate of the carbon monoxide gas may more preferablybe 1 NL/g-active species⋅minute or more. By setting the flow rate of thecarbon monoxide gas to be within this range, the CNFs can be producedwith high synthesis activity. Here, the high synthesis activityspecifically means 10 g-CNF/g-active species·h (hour) or more. There isno particular upper limit on the flow rate of the carbon monoxide gas,but if it exceeds 200 NL/g-active species⋅minute, the flow rate of thegas is too high and the utility cost for preheating is increased, whichis not preferable. In addition, the synthesis activity may be decreased.Therefore, the flow rate of the carbon monoxide gas may more preferablybe 100 NL/g-active species⋅minute or less.

Prior to the reaction, the catalyst may be reduced with hydrogen orother reducing gases. In this case, hydrogen or other reducing gases maybe optionally diluted with an inert gas such as nitrogen. The reductionis preferably carried out at the same temperature as the reactiontemperature or in a temperature range of about ±200° C. of the reactiontemperature. The reduction of the catalyst prior to the reaction mayincrease or stabilize the catalytic activity.

Method for Removing Residual Catalyst

The catalyst(s) is/are preferably removed from the CNF compositeproduced according to the present invention, in order to improve thepurity, because, for example, when the carbon nanofiber compositeaccording to the present invention is used as a conductive material forelectrodes of various batteries such as lithium ion secondary batteries,it may be necessary to remove residual catalyst. In this case, a knownmethod may be utilized. However, removal of residual catalyst in anaqueous solution such as a residual catalyst dissolution treatment withan acid or the like cause a problem that the carbon nanofiber compositeis aggregated and the dispersibility is decreased during drying of thecarbon nanofiber composite, or removal of the residual catalystcontained in the carbon is not sufficient. The carbon nanofibercomposite according to the present invention can be subjected a heattreatment at a temperature of 1500° C. or higher and 2500° C. or lowerin an inert gas to efficiently remove the residual catalyst.Specifically, according to the present invention, it is possible toremove 80% by mass or more of the residual catalyst contained, andpreferably 90% by mass or more, that is, the target is a residualcatalyst removal rate of 80% by mass or more, and preferably 90% by massor more. The time of the heat treatment depends on the amount and thestate of existence of residual catalyst, and may be optionally changed.It may be usually 30 minutes or more and 6 hours or less. A knownheating furnace can be used for the heat treatment, and a graphitizationfurnace may be suitably used. Examples of the inert gas include argon,nitrogen, helium, and xenon.

Carbon Nanofiber (CNF) Composite

The CNF composite according to the present invention includes carbonnanofibers and at least one particulate carbon material. In oneembodiment, the CNF composite according to the present invention canhave a structure in which carbon nanofibers and at least one particulatecarbon material are intertwined with each other.

The CNF composite according to the present invention can becharacterized by the following features:

The CNFs may be contained in any proportion (CNF content) in the CNFcomposite according to the present invention, and the CNF content maypreferably be in a range of from 10 to 90% by mass. If the content ismore than 90% by mass, the effect of improving the dispersibility by theparticulate carbon material (fluidizing dispersant) may be limited. Ifthe content is less than 10% by mass, the effect of CNFs themselves onthe physical properties (impartment of conductivity, and the like) maybe limited. The CNF content may more preferably be 20% by mass or more,and still more preferably 30% by mass or more, and even more preferably40% by mass or more, and even more preferably 50% by mass or more, andstill more preferably 60% by mass or more, and still more preferably 70%by mass or more, and still more preferably 80% by mass or more, in termsof producing the effect of CNFs themselves on the physical properties.The CNF content may more preferably be 85% by mass or less, in terms ofimproving the dispersibility. Here, the CNF content (% by mass) isrepresented by the equation: Mass of CNF/(Mass of CNF+Mass ofParticulate Carbon Material)×100.

The CNF composite according to the present invention can exhibit variousconductivities (volume resistivities) depending on the type of thefluidizing dispersant made of the carbon material to be used and theproportion of the fluidizing dispersant contained. As a preferredexample, when carbon black is used as the fluidizing dispersant, thevolume resistivity measured under a load of 9.8 MPa is generally 0.1Ω·cm or less. When graphite or graphene is used as the fluidizingdispersant, the volume resistivity measured under a load of 9.8 MPa isgenerally 0.05 Ω·cm or less.

The CNF composite according to the present invention, more preferablythe MWCNT composite, is characterized by exhibiting high dispersibility.

The CNF composite according to the present invention can exhibit higherdispersibility as compared with the conventional commercial CNFs(MWCNTs). Furthermore, the “higher dispersibility” as used herein meansthat a higher dispersible CNF composite can be obtained as compared withCNFs obtained by a method other than that of the present invention, forexample without using the fluidizing dispersant according to the presentinvention, or as compared with a CNF composite obtained withoutmechanical stirring. The dispersibility may be evaluated by any knownmethod. For example, the evaluation method includes a method of visuallyobserving a state of a dispersion film formed on a glass wall surface ofa vial after stirring a CNF dispersion in the glass vial; a particlesize distribution measurement method; a particle size gauge (particlegauge) method; Turbi Online (trade name from EKO Instruments); aprecipitation test, a centrifugal sedimentation test, and the like.Preferably, the particle size distribution measurement method is used.Pretreatment for measurement performed prior to the dispersibilityevaluation may vary depending on methods and conditions, conditioning ofthe apparatus, and the like. Therefore, it can be demonstrated that theCNF composite according to the present invention can exhibit higherdispersibility by suitably selecting and optimizing those conditions andthe like, as compared with the above commercially available CNFs, andCNFs or a CNF composited obtained by a method other than that of thepresent invention. A non-limiting example includes the followingmethods:

Standardized Pretreatment for Measurement

6 mL of an aqueous solution of 0.1% by mass of sodium carboxymethylcellulose (CMCNa) is prepared, and a dispersion solution is preparedsuch that a concentration of the CNF composite in the aqueous solutionis 0.1% by mass. Using an ultrasonic homogenizer (for example, Smurt NR50-M available from Microtec Co., Ltd.;⋅frequency: 20 kHz; output: 50W), the dispersion solution was subjected to ultrasonic irradiation for40 seconds under conditions of an auto power mode and output of 50% (25W) to prepare a sample solution suspended and homogenized.

Particle Size Distribution Measurement

The sample solution is then subjected to particle size distributionmeasurement according to the laser diffraction/scattering method (ISO13320: 2009).

The samples (the CNF composites or comparative samples) used in thismeasurement are not subjected to any dispersion treatment other than theabove standardized pretreatment for measurement. Here, the “dispersiontreatment other than the above standardized pretreatment formeasurement” means any conventional dispersion treatment that willaffect the dispersibility, including a manual dispersion treatment witha mortar or the like, a mechanical dispersion treatment such as a jetmill, a bead mill, a ball mill, an emulsification disperser and thelike, or a dispersion treatment with ultrasonic waves, such as anultrasonic homogenizer or an ultrasonic cleaning machine.

In the above measurement, the CNF composite according to the presentinvention may have, in one embodiment, 70% by volume or more, forexample from 70 to 95% by volume, and preferably 80% by volume or more,for example from 80 to 95% by volume, of particles each having aparticle diameter of 1 μm or less. In the above measurement, the CNFcomposite according to the present invention may exhibit highdispersibility such that, in one embodiment, a median diameter D50 byvolume is 1.0 μm or less, for example, from 0.2 to 1.0 μm, andpreferably 0.5 μm or less, for example from 0.2 to 0.5 μm.

Resin Composition

The CNF composite according to the present invention can be suitablyused for a conductive resin composition, because the CNF composite hashigh dispersibility. Such a conductive resin composition can be used asan antistatic material for electronic component packaging materials,clean room flooring materials, combustible fuel tank materials,electricity removing rolls of copying machines and the like, or as aconductive material such as a power cable covering material, a surfaceswitch, a connector, a print circuit and the like; a resistor for a roofmaterial and a carpet; an motor vehicle exterior material forelectrostatic painting; or an electronic component housing material forelectromagnetic wave shielding.

The resin used for the conductive resin composition includes, but notlimited to, thermoplastic resins or thermosetting resins. Examples ofthe thermoplastic resin include, for example, polyvinyl chloride resinswith or without a plasticizer; polyolefin resins such as low densitypolyethylene, high density polyethylene, linear low densitypolyethylene, polyolefinic elastomer (POE), various stereoregular oratactic polypropylene, a polyethylene-propylene copolymer (EPR) andcyclic polyolefin; styrene based resins such as polystyrene, anacrylonitrile-butadiene-styrene (ABS) resin and an acrylonitrile-styrene(AS) resin; poly(meth)acrylic resins such as poly(methyl acrylate),poly(methyl methacrylate), poly(ethyl acrylate), poly(ethylmethacrylate), poly(acrylic acid) and poly(methacrylic acid);polyacrylonitrile resins; polyester resins such as polybutyleneterephthalate and polyethylene terephthalate; polylactic acid resins;ionomer resins; polycarbonate resins; polyamide resins such as variousnylons; polyacetal resins; polyphenylene ether resins; modifiedpolyphenylene ether resins; polyarylate resins; polysulfone resins;polyetherimide resins; polyether sulfone resins; polyphenylene sulfideresin; polyether ether ketone resins; polyether ketone resins; polyamideimide resins; thermoplastic polyimide resins; liquid crystal polyesterresins; styrene-diene block copolymers such as SBS, SIS SEBS and SEPS;petroleum resins; and polymer alloys thereof. Examples of thethermosetting resin include various epoxy resins, cyanate resins,benzoxazine resins, and bismaleimide resins.

The CNF composite according to the present invention having improveddispersibility provides advantages that the composite can moderateconditions for kneading the thermoplastic resin, such as low shear rateand a short period of time, and can significantly reduce the costs forproducing the resin composition. Further, the CNF composite according tothe present invention having excellent conductivity can reduce theamount of conductive filler (CNF composite) required for imparting thesame conductivity to the resin composition, resulting in a reduction incost, as well as it can prevent deterioration of mechanical propertiesand deterioration of formability. Further, the composite havingexcellent dispersibility provides an advantage that lumps, aggregatesand the like of CNFs are reduced, thereby easily providing highmechanical properties. Even if the resin is the thermosetting resin,there is the same advantages as described above when producing theuncured resin composition before curing.

The method for producing the conductive resin composition according tothe present invention includes, but not particularly limited to, anyknown suitable blending methods. For example, the melt blending may becarried out in a single screw or twin screw extruder, a Banbury typemixer, a plastomill, a co-kneader, a heating roll or the like. Prior tothe melt blending, each raw material may be uniformly mixed by aHenschel mixer, a ribbon blender, a super mixer, a tumbler or the like.The temperature for the melt blending is not particularly limited, butmay be generally from 100 to 300° C., and preferably from 150 to 250° C.When producing a molded article of the conductive resin according to thepresent invention, the method for producing the molded article that canbe used includes, but not particularly limited to, in general, a pressmolding method, an extrusion molding method, a roll molding method, aninjection molding method or a transfer molding method.

Dispersion, Ink, Paint

In general, a dispersion, an ink and a paint (hereinafter referred to asa dispersion and the like) in which a conductive material is dispersedare used for producing a substrate material for electrodepositioncoating, a conductive adhesive for IC, a tray for low voltage parts anda cover sheet. It is also used as a raw material of a conductive agentfor electrodes of various batteries including lithium ion secondarybatteries. The CNF composite according to the present invention can alsobe suitably used for producing the dispersion and the like, because thecomposite has high dispersibility. The dispersion and the like areobtained by adding the conductive material (CNF composite), a knowndispersing agent and optionally a binder to a suitable solvent,mechanically dispersing them using a jet mill, a bead mill, a ball mill,an emulsifying disperser or the like, or dispersing them by means ofultrasonic waves such as an ultrasonic homogenizer or an ultrasonicwashing machine. In this case, the use of the CNF composite according tothe present invention provides advantages that a relatively moderatedispersion is sufficient because of the high dispersibility so that thecost of the dispersion treatment can be reduced, and the dispersion andthe like having higher concentration can be easily obtained. Further,the resulting dispersion and the like contain less amount of lumps,aggregates and the like of CNFs, which are suitable for thisapplication.

Examples

Hereinafter, the present invention will be described with reference toExamples, but these Examples are not intended to limit the presentinvention.

Cobalt-Magnesium Oxide Supported Catalyst: Catalyst A (CatalystPreparation Example 1)

6.17 g of cobalt nitrate hexahydrate (3N5, available from KANTO CHEMICALCO., INC.) was weighed and dissolved in 30 g of a mixed solvent ofdistilled water and ethanol having a mass ratio of 2:1. To the aqueouscobalt nitrate solution was added 2.5 g of magnesium oxide (DENMAG®KMAOH-F, available from Tateho Chemical Industries Co., Ltd.) having aspecific surface area of 0.61 m²/g, and the mixture was stirred at 50°C. for 1 hour while maintaining the mixture in a hot water bath. Afterstirring, water was evaporated by means of an evaporator. The resultingsolid component was dried under vacuum at 60° C. for 24 hours and thencalcined at 400° C. for 5 hours. After the calcination treatment, theresulting solid component was pulverized in an agate mortar to obtain acobalt-magnesium oxide supported catalyst on which 50% by mass of cobaltmetal was supported.

Cobalt-Titanium Oxide Supported Catalyst: Catalyst B (CatalystPreparation Example 2)

2.5 g of titanium oxide (AEROXIDE® “TiO₂ P25”, available from NIPPONAEROSIL CO., LTD., specific surface area: 52 m²/g) having a molar ratioof an anatase structure to a rutile structure of 80 to 20, and 2.4 g ofcobalt nitrate hexahydrate (3N5, available from KANTO CHEMICAL CO.,INC.), were dissolved in 30 mL of distilled water. The resultingsolution was set in a rotary evaporator (N1000, available from TOKYORIKAKIKAI CO., LTD.), heated to 50° C. in a water bath and stirred for 1hour. After removing water, it was further dried under vacuum at 60° C.for 12 hours to obtain a solid component. The resulting solid componentwas transferred to a ceramic crucible and calcined in a muffle furnace(FO 200 from YAMATO SCIENTIFIC CO., LTD.) in air at 400° C. for 5 hoursto obtain a cobalt-titanium oxide supported catalyst having a cobaltsupported rate of 20%.

Iron/Nickel-Graphite Supported Catalyst: Catalyst C (CatalystPreparation Example 3)

Iron (II) acetate and nickel (II) acetate tetrahydrate were weighed suchthat the total supported rate of iron (as a metal component) and nickel(as a metal component) per 1 g of J-SP (high purity graphite powderavailable from Nippon Graphite) was 5% by mass and a mass ratio of ironto nickel was 5:5, and 6 g of distilled water and 6 g of ethyl alcohol(a mass ratio of distilled water:ethyl alcohol=1:1) per 1 g of thegraphite powder were added, and these substances were sufficientlydissolved and dispersed. An eggplant-shaped flask containing theresulting solution was attached to a rotary evaporator and stirred whilerotating it in a water bath at 50° C. for 1 hour. The pressure of therotary evaporator was then reduced and the solvent was removed. Afterremoving the solvent, the flask was removed from the evaporator anddried in a vacuum dryer at 60° C. for 15 hours or more. The resultingcatalyst was scraped out from the flask using a spatula and transferredto an agate mortar, and the agglomerated catalyst was pulverized toobtain an iron-nickel/graphite supported catalyst having a supportedrate of 5% by mass and a mass ratio of iron to nickel of 5:5. Thecatalyst was stored in a dry state.

An exact amount of active species in each catalyst was determined bycarrying out chemical analysis of the catalyst.

Measurement of Specific Surface Area

The specific surface area of each CNF composite was determined by theBET one point method according to JIS K 6217-2: 2001 using Macsorb HMmodel-1201 from MOUNTECH Co., Ltd.

Measurement of Volume Resistivity (Powder Resistivity)

The volume resistivity of each CNF composite was measured according tothe four point probe method under a condition of a load of 9.8 MPa in anatmosphere at 23° C. and a relative humidity of 50% using Loresta GPfrom Mitsubishi Chemical Analytech, Co., Ltd.: a powder resistivitymeasurement system, model MCP-PD 51. The measurement was carried outusing 100 mg of each sample.

Evaluation of Dispersibility: Measurement of Particle Size Distributionby Laser Diffraction/Scattering Method

The ratio of dispersed particles of 1 μm or less and the median diameterD50 were measured by means of a particle size distribution measuringapparatus (LS 13320 Universal Liquid Module from BECKMAN COULTER INC.).

Prior to the measurement of the ratio of dispersed particles of 1 μm orless and the median diameter, the particle size distribution measuringapparatus was tested. When a value of the median diameter D50 obtainedby measurement of each test sample as described below satisfied all ofthe following conditions, the measurement accuracy of the apparatus wasconsidered to be acceptance, and the particle size distributionmeasurement was carried out in Examples and Comparative Examples.

Preparation of Aqueous Dispersion Medium

0.10 g of sodium carboxymethyl cellulose (hereinafter referred to as“CMCNa”) was added to 100 mL of distilled water and stirred at normaltemperature for 24 hours or more to dissolve it to prepare an aqueousdispersion medium containing 0.1% by mass of CMCNa.

[Preparation of Aqueous CMCNa Solution]

2.0 g of CMCNa was added to 100 mL of distilled water, and stirred anddissolved at normal temperature for 24 hours or more to prepare anaqueous solution containing 2.0% by mass of CMCNa.

Preparation of Test Sample and Testing

(1) Testing with Polystyrene Dispersion

An aqueous dispersion LATRON 300 LS (median diameter D50: 0.297 μm) forconfirming measurement accuracy, attached to a particle sizedistribution measuring apparatus (LS 13 320 Universal Liquid Module fromBECKMAN COULTER INC.), was used.After setting optical models to refractive indices of 1.600 forpolystyrene and 1.333 for water, respectively, and washing the module,about 1.0 mL of the above aqueous CMCNa solution was filled. Afterperforming offset measurement, optical axis adjustment and backgroundmeasurement at a pump speed of 50%, the particle size distributionmeasurement was carried out by adding the LATRON 300 LS to a particlediameter analyzer such that a relative concentration indicating apercentage of light scattered by particles outside the beam was from 8to 12% or PIDS was from 40% to 55%. A graph of % by volume versus aparticle size (particle diameter) was obtained and accuracy wasconfirmed. It was confirmed that the median diameter D50 value obtainedby the measurement was within 0.297 μm±0.018 μm, the median diameter D10value was within 0.245 μm±0.024 μm, and the median diameter D90 valuewas within 0.360 μm±0.036 μm.

(2) Testing with Alumina Dispersion

0.120 g of each of alumina LS-13 (median diameter D50: 45 μm) availablefrom Denka Company Limited and alumina AS-50 (median diameter D50: 6.7μm) available from Showa Denko K.K. was weighed in a vial, 12.0 g of theaqueous dispersion medium as described above was added, and the vial wasthoroughly shaken to prepare an aqueous alumina dispersion.

After setting the optical models to refractive indices of 1.768 foralumina and 1.333 for water, respectively, and washing the module, about1.0 mL of the above aqueous CMCNa solution was filled. After performingoffset measurement, optical axis adjustment and background measurementat a pump speed of 50%, the particle size distribution measurement wascarried out by adding the prepared aqueous alumina dispersion to theparticle diameter analyzer such that a relative concentration indicatinga percentage of light scattered by particles outside the beam was from 8to 12% or PIDS was from 40% to 55%. A graph of % by volume versus aparticle size (particle diameter) was obtained and accuracy wasconfirmed. It was confirmed that the median diameter D50 value obtainedby the measurement was within 48.8 μm±5.0 μm for LS-13, and within 12.6μm±0.75 μm for AS-50.

Standardized Pretreatment for Measurement

6.0 mg of each CNF composite was weighed in a vial, and 6.0 g of theaqueous dispersion medium was added thereto. An ultrasonic homogenizerSmurt NR-50 (from MICROTEC CO., LTD., output: 50 W) was used for thepretreatment for measurement.

It was confirmed that there was no deterioration of a chip which wasattached to the tip of the ultrasonic homogenizer and which generatedoscillation, and the chip was adjusted such that the chip was dipped ina depth of 10 mm or more from the liquid level of a sample to betreated. The chip in which a total of ultrasonic generation time waswithin 30 minutes were used. In the homogenizer, TIME SET (irradiationtime) was set to 40 seconds, POW SET was set to 50%, START POW was setto 50% (output of 50%), and homogenization by ultrasonic irradiation wasperformed by auto power operation with constant output power to preparean aqueous CNF dispersion.

Measurement of Particle Size Distribution for CNF Composite

Using the aqueous CNF composite dispersion prepared by the above method,the ratio of the dispersed particles of 1 μm or less in the CNFs and themedian diameter were measured according to the following method. Aftersetting optical models of LS 13320 Universal Liquid Module (inaccordance with ISO 13320: 2009) to refractive indices of 1.520 for CNFand 1.333 for water, respectively, and washing the module, about 1.0 mLof the aqueous CMCNa solution was filled. After performing offsetmeasurement, optical axis adjustment and background measurement at apump speed of 50%, the prepared aqueous CNF composite dispersion wasadded to a particle diameter analyzer such that a relative concentrationindicating a percentage of light scattered by particles outside the beamwas from 8 to 12% or PIDS was from 40% to 55%, ultrasonic irradiationwas performed at 78 W for 2 minutes by a device attached to the particlediameter analyzer (pretreatment for measurement), air bubbles wereeliminated by circulation for 30 seconds, and particle size distributionmeasurement was then carried out. A graph of % by volume versus aparticle size (particle diameter) was obtained, and the existence ratioof the dispersed particles of 1 μm or less and the median diameter D50value were determined.

For the measurement, three measuring samples were sampled at differentpositions for one sample of the CNF composites, and the above particlesize distribution measurement was carried out three times for onesample. An average value of seven values obtained by excluding themaximum value and the minimum value from nine values of each of thevolume fractions and the median diameter D50 values of particles eachhaving a particle diameter of 1 μm or less was obtained, and the averagevalue was considered to be the measured value.

Particulate Carbon Material (Fluidizing Dispersant)

The following materials were used as carbon black. As acetylene black,DENKA BLACK “HS-100 (trade name)” (a specific surface area of 39 m²/g),“FX-35 (trade name)” (a specific surface area of 133 m²/g), and“Powdered Product (trade name)” (a specific surface area of 68 m²/g)were used, all of which were available from Denka Company Limited. AsKetjen black, EC 300J (a specific surface area of 800 m²/g) availablefrom Lion Corporation was used. As furnace black, SUPER P Li (a primaryparticle size of 40 nm, a specific surface area of 62 m²/g) availablefrom Timcal Corporation was used.

The volume fraction and the median diameter D50 value of the particleseach having a particle diameter of 1 μm or less were obtained byperforming the above “Dispersibility Evaluation: Particle SizeDistribution Measurement by Laser Diffraction/Scattering Method” in thesame method on these fluidizing dispersants. The results are shown inTable 1. However, the optical model during the measurement of theparticle size distribution of the fluidizing dispersant was set to arefractive index of 1.520 for acetylene black. Each of the carbon blacksused was a product satisfying the aspect ratio condition as theparticulate carbon material described above.

Reactor for Synthesizing CNF

A horizontal rotary reactor 100 schematically shown in FIG. 1 wasconnected to a commercially available rotary evaporator rotating device(N-1110 V from TOKYO RIKAKIKAI CO., LTD.) (not shown), and batchwisereaction was carried out. The reactor 100 is composed of a fixingportion 104 (non-rotating; made of Pyrex® glass) and a rotating portion103 (made of cylindrical quartz glass). Further, at the center of thereactor 100 is a non-rotating gas introduction portion 105 (tubular witha diameter of 12 mm) connected to the fixing portion 104. The rotatingportion 103 has a reaction portion 107 (a length of about 20 cm, adiameter of 5 cm) with a stirring blade 106 on the inner wall of thecylindrical portion at the tip of the rotating portion 103. Thearrangement of the stirring blade 106 is as shown in the end view takenalong the line A-A′ in FIG. 2. The fixing portion 104 is provided with agas introduction pipe 108 vertically connected to the gas introductionportion 105 and a thermocouple introduction pipe 109 straightlyconnected to the gas introduction portion 105. Sealed thermocouples 110are inserted from the thermocouple introduction pipe 109 and areinverted to a degree of 180 on the outer side of the outlet of the gasintroduction portion 105. Temperature measuring portions of thethermocouples measure a temperature of a gas phase in the reactionportion 107 on the outer side of the gas introduction portion 105. Threethermocouples 110 are present and measure temperatures of the center,the right end portion and the left end portion of the reaction portion107. The entire reaction portion 107 can be uniformly heated byindependently controlling three electric furnaces of three-zonehorizontal type tubular electric furnaces (not shown) arranged on theouter periphery of the reaction portion 107. A gas exhaust pipe 111connected to the outer peripheral portion of the fixing portion 104 isinstalled, and an exhaust gas from the reaction portion 107 isdischarged from the gas exhaust pipe 111.

The reaction was carried out by charging predetermined amounts of acatalyst and a fluidizing dispersant in the reaction portion 107 of thereactor 100 and rotating the rotating portion 105 while allowing a rawmaterial gas to flow from the gas introduction portion 108, through thegas introduction portion 105 and the reaction portion 107, and into thegas exhaust pipe 111 in a state where the reactor 100 was inclinedhorizontally or slightly downward.

Table 2 shows the catalyst, the amount of the active species (as metalcontent), the fluidizing dispersant and its amount used in each ofExamples and Comparative Examples, the reaction temperature, thecomposition of the raw material gas, the partial pressure of the carbonmonoxide gas, the flow rate of the carbon monoxide gas, the reactiontime, the mass of the produced CNFs, the synthesis activity, the bulkdensity, the volume resistivity and the specific surface area of theresulting CNF composite, the CNF content in the CNF composite, theproportion (% by volume) of particles each having a particle diameter of1 μm or less, and the median diameter D50.

Here, the mass of the produced CNFs is a value obtained by subtractingthe total mass of the catalyst and the fluidizing dispersant from themasses of all of the recovered products. The CNF content in the CNFcomposite was determined by the following equation:

CNF Content in CNF Composite=Mass of Produced CNF/(Mass of ProducedCNF+Mass of Particulate Carbon Material)×100(%).

Example 1

To the reactor was added the catalyst A (cobalt-magnesium oxidesupported catalyst) obtained in Catalyst Preparation Example 1 in anamount of 24 mg based on the mass of the active species cobalt metal,and acetylene black (Denka Black, HS-100) in an amount of 1.5 g. Anitrogen gas flowed at room temperature under atmospheric pressure whilerotating the rotating portion at 30 rpm and sufficiently replaced in thereactor, and the temperature rising was then started. When thetemperature reached 605° C. (a temperature of minus 50° C. relative tothe reaction temperature), reducing gases (90 m/min of nitrogen and 200m/min of hydrogen at actual flow rates) instead of the nitrogen gasflowed under atmospheric pressure. After reaching the reactiontemperature of 655° C., the flowing of the reducing gases was stoppedafter 20 minutes from the start of flowing of the reducing gases, andreaction gases (carbon monoxide: 13 NL/g-active species⋅minute,hydrogen: 2.3 NL/g-activity species⋅minute) flowed under atmosphericpressure. The reaction was carried out for 3 hours while maintaining thetemperature at 655° C. The raw material gases were stopped and thereactor was cooled while replacing the raw material gases with thenitrogen gas.

Examples 2 to 16

The reaction was carried out under the same conditions as those ofExample 1 with the exception that the conditions shown in Table 2 werechanged.

Comparative Example 1

The reaction was carried out under the same conditions as those ofExamples 1, 2 and 3, with the exception that a small amount of CNF wasused in place of the fluidizing dispersant of the present invention.This CNF is obtained by Reference Example and has high dispersibility.

Comparative Example 2

The reaction was carried out under the same conditions as those ofExample 4, with the exception that a small amount of CNF was used inplace of the fluidizing dispersant of the present invention. This CNF isobtained by Reference Example and has high dispersibility.

Comparative Example 3

The reaction was carried out in the same method as that of Example 4 andComparative Example 2, with the exception that the fluidizing dispersantor the CNFs of the present invention were not used, and the reaction wascarried out using only the catalyst. Many of the resulting CNFs wererecovered in a state where they adhered to the wall surface of thereactor. It is considered that most of the catalyst and produced CNFsdid not flow during the reaction.

Comparative Example 4

The reaction was carried out under the same conditions as those ofExample 1 with the exception that the conditions shown in Table 2 werechanged. The specific change is that 10 g of zirconia beads (a diameterof 2 mm) was used as a fluidizing dispersant in place of the fluidizingdispersant of the present invention. The resulting product was passedthrough a sieve to obtain CNFs alone.

Comparative Example 5

The reaction was carried out under the same conditions as those ofExample 4, with the exception that the reactor was not rotated.

TEM observation demonstrated that CNFs in the CNF composite obtained ineach Example and Comparative Example were MWCNTs. Further, it was alsoconfirmed that the average outer diameter and the aspect ratioindicating the ratio of the fiber length to the average outer diametersatisfied the definition of CNF according to the present invention. Byway of example, FIGS. 3, 4 and 5 show TEM photographs of CNFs in thecomposites obtained in Example 1, Example 8, and Example 9,respectively.

When comparing Examples 1, 2, and 3 with Comparative Example 1, it isunderstood that each CNF composite obtained by carrying out the reactionusing the fluidizing dispersant according to the present invention as afluidizing dispersant has higher dispersibility for the CNF content ofeach CNF composite, in view of its volume fraction of dispersedparticles of 1 μm or less and its value of median diameter D50. On thecontrary, in Comparative Example 1, CNFs having lower dispersibilitywere obtained. When comparing Example 4 with Comparative Example 2, theCNF composite obtained by carrying out the reaction using the fluidizingdispersant according to the present invention as a fluidizing dispersantexhibits higher dispersibility as well. On the contrary, in ComparativeExample 2, CNFs having lower dispersibility are obtained. As shown inExamples 5 and 6, even if the fluidizing dispersant used is changed toother acetylene black that meets the conditions of the presentinvention, a highly dispersible CNF composite can be obtained as well.As shown in Examples 7 to 14, it was confirmed that each of the CNFcomposites obtained by changing the catalyst, the temperature, the flowrates of the gases, and the amount of the fluidizing dispersant used(that is, the CNF content in the CNF composite) within the scope of thepresent invention exhibited higher dispersibility. In Examples 15 and16, it was confirmed that even if the fluidizing dispersant used waschanged to Ketjen black or furnace black which are other carbon blacksmeeting the conditions of the present invention, the CNF compositeshaving higher dispersibility could be obtained as well. As shown inComparative Example 3, when no fluidizing dispersant was used, thefluidity of the catalyst or CNFs in the reactor was deteriorated. Also,the dispersibility of the resulting CNFs was poor. In ComparativeExample 4, CNFs were produced under substantially the same conditions asthose of Examples 4 and 5 with the exception that zirconia beads wereused, but the CNFs obtained under the conditions had poordispersibility. FIG. 6 shows the results of the dispersibilityevaluation (a graph showing the frequency distribution of % by volumeversus a particle diameter) of the CNF composite obtained in Example 1.FIG. 7 shows the results of the dispersibility evaluation (a graphshowing the frequency distribution of % by volume versus a particlediameter) of the CNF composite obtained in Comparative Example 1.Comparative Example 5 shows the results obtained by carrying out thereaction without rotating the reactor, but under the conditions, the CNFcomposite having poor dispersibility was obtained.

Reference Example

Under the conditions shown in Table 2, 10 g of zirconia beads (adiameter of 2 mm) was used as a fluidizing dispersant in place of thefluidizing dispersant according to the present invention. In this case,the product obtained is MWCNTs. When using the catalyst, CNFs havinggood dispersibility can be synthesized at a temperature of 700° C. TheCNFs were used as a fluidizing dispersant for Comparative Examples 1 and2.

Comparison of CNF syntheses at the same temperature, that is, comparisonof Example 10 with Reference Example or Example 4 with ComparativeExample 4 shows that the activity of CNF synthesis per unit activespecies is significantly higher in Example 10 and Example 4. The reasonwould be that the fluidizing dispersant according to the presentinvention enables more efficient contact of the raw material gas withthe catalyst due to the dispersibility and fineness of the fluidizingdispersant, as compared with the ceramic ball fluidizing dispersant.

In view of the foregoing, it is understood that the growth of CNFsthrough the mechanical stirring in the coexistence of the fluidizingdispersant such as carbon black, the catalyst and the raw material gascontaining the carbon-containing compound can prevent the CNFs frombeing entangled and allow the highly dispersed CNF composite material tobe efficiently obtained. FIG. 8 shows a SEM photograph of the CNFcomposite obtained in Example 2. As can be seen from the FIG. 8,particulate acetylene black and fibrous CNFs uniformly commingles witheach other.

Removal of Residual Catalyst Example 17

Using the CNF composite obtained in Example 4, the catalyst was removedby a heat treatment. 1.0 g of CNFs was placed in a graphite crucible,the crucible was set in a graphitizing furnace from Thermonic Co., Ltd,without a lid, and the temperature was increased at 25° C./min in anitrogen atmosphere and maintained at 1700° C. for 3 hours. An amount ofthe residual catalyst for the sample after the heat treatment wasquantified. The results are shown in Table 3.

Example 18

The heat treatment was carried out in the same method as that of Example17, with the exception that the heat treatment conditions were changedto 2000° C. and 1 hour.

Examples 19 and 20

Using the CNF composite obtained in Example 7, each heat treatment wascarried out in the same method as in each of Examples 17 and 18.

Comparative Examples 4 and 6

Each heat treatment was carried out in the same method as in each ofExamples 17 and 19 with the exception that the heat treatment conditionswere changed to 1200° C. and 3 hours.

Comparative Examples 5 and 7

Using 0.2 g of each of the CNF complexes obtained in Examples 4 and 7,the residual catalyst was removed by the following acid dissolutiontreatment without any heat treatment. That is, each CNF composite wasadded to 300 mL of an aqueous 1N hydrochloric acid solution, stirred for24 hours, then collected by filtration and washed with a sufficientamount of distilled water.

As can be seen from the results shown in Table 3, it is difficult tosufficiently remove the catalyst under the heat treatment conditionsshown in Comparative Examples. Further, in the removal of residualcatalyst by the acid treatment, the removal rate of the residualcatalyst is higher, but the removal of the active species metal is notsufficient. In contrast, as shown in Examples, the conducting of theheat treatment at a temperature of 1500° C. or more and 2500° C. or lessin an inert gas allows efficient residual catalyst removal which matchesto the object of the present invention.

TABLE 1 Median Particles Diameter Fluidizing of 1 μm or less D50Dispersant Name of Product [% by volume] [μm] Acetylene Black DENKABLACK HS-100 95 0.4 Acetylene Black DENKA BLACK FX-35 87 0.4 AcetyleneBlack DENKA BLACK 95 0.4 Powdered Product Ketjen Black EC 300J 85 0.4Furnace Black SUPER P 95 0.4

TABLE 2 Carbon Carbon Monoxide Type of Catalyst Raw Material MonoxideGas Flow Rate Amount of Fluidizing Dispersant Reaction Gas Gas Partial[NL/g-active Reaction Active Used Temperature Composition Pressurespecies · Time Test No. Catalyst Used Species/mg Type Amount/g [° C.](Molar Ratio) [MPa] min] [h] Example 1 Catalyst 24 Acetylene Black 1.5655 CO/H₂/N₂ = 0.086 13 3 Preparation HS-100 85/15/0 Example 1 Co/MgOExample 2 Catalyst 24 Acetylene Black 1 655 CO/H₂/N₂ = 0.086 13 3Preparation HS-100 85/15/0 Example 1 Co/MgO Example 3 Catalyst 24Acetylene Black 3.6 655 CO/H₂/N₂ = 0.086 13 3 Preparation HS-100 85/15/0Example 1 Co/MgO Example 4 Catalyst 77 Acetylene Black 0.65 655 CO/H₂/N₂= 0.086 6 1 Preparation HS-100 85/15/0 Example 1 Co/MgO Example 5Catalyst 77 Acetylene Black 0.65 655 CO/H₂/N₂ = 0.086 6 1 PreparationFX-35 85/15/0 Example 1 Co/MgO Example 6 Catalyst 77 Acetylene Black0.65 655 CO/H₂/N₂ = 0.086 6 1 Preparation Powdered Product 85/15/0Example 1 Co/MgO Example 7 Catalyst 40 Acetylene Black 2.0 610 CO/H₂/N₂= 0.086 25 1 Preparation HS-100 85/15/0 Example 1 Co/MgO Example 8Catalyst 27 Acetylene Black 1.4 610 CO/H₂/N₂ = 0.052 53 1 PreparationHS-100 51/22/27 Example 2 Co/TiO2 Example 9 Catalyst 14 Acetylene Black1.3 610 CO/H₂/N₂ = 0.052 53 1 Preparation HS-100 51/22/27 Example 3Fe—Ni/C Example 10 Catalyst 77 Acetylene Black 0.65 700 CO/H₂/N₂ = 0.0866 1 Preparation HS-100 85/15/0 Example 1 Co/MgO Example 11 Catalyst 77Acetylene Black 0.65 750 CO/H₂/N₂ = 0.086 6 1 Preparation HS-100 85/15/0Example 1 Co/MgO Example 12 Catalyst 40 Acetylene Black 2 610 CO/H₂/N₂ =0.086 25 1 Preparation HS-100 85/15/0 Example 1 Co/MgO Example 13Catalyst 40 Acetylene Black 1.6 610 CO/H₂/N₂ = 0.086 25 1 PreparationHS-100 85/15/0 Example 1 Co/MgO Example 14 Catalyst 20 Acetylene Black1.2 655 CO/H₂/N₂ = 0.086 51 1 Preparation HS-100 85/15/0 Example 1Co/MgO Example 15 Catalyst 77 Ketjen Black 0.65 655 CO/H₂/N₂ = 0.086 6 1Preparation EC 300J 85/15/0 Example 1 Co/MgO Example 16 Catalyst 77Furnace black 0.65 655 CO/H₂/N₂ = 0.086 6 1 Preparation SUPER P 85/15/0Example 1 Co/MgO Comparative Catalyst 24 CNF 0.023 655 CO/H₂/N₂ = 0.08613 3 Example 1 Preparation 85/15/0 Example 1 Co/MgO Comparative Catalyst77 CNF 0.075 655 CO/H₂/N₂ = 0.086 6 1 Example 2 Preparation 85/15/0Example 1 Co/MgO Comparative Catalyst 77 Non 0 655 CO/H₂/N₂ = 0.086 6 1Example 3 Preparation 85/15/0 Example 1 Co/MgO Comparative Catalyst 120Zirconia Beads 10 655 CO/H₂/N₂ = 0.086 6 1 Example 4 Preparation 85/15/0Example 1 Co/MgO Comparative Catalyst 77 Acetylene Black 0.65 655CO/H₂/N₂ = 0.086 6 1 Example 5* Preparation HS-100 85/15/0 Example 1Co/MgO Reference Catalyst 120 Zirconia Beads 10 700 CO/H₂/N₂ = 0.086 6 1Example Preparation 85/15/0 Example 1 Co/MgO Percentage of ParticlesSpecific Having Particle Median Mass of Synthesis Activity VolumeSurface CNF Content in Diameter of Diameter Produced CNF [g-CNF/g-activeBulk Density Resistivity Area CNF Composite 1 μm or less D50 Test No.[g] species · h] [g/cm³] [Ω · cm] [m²/g] [wt %] [vol %] [μm] Example 11.41 20 0.082 0.024 134 48 83 0.4 Example 2 1.68 23 0.074 0.016 140 6383 0.4 Example 3 1.33 19 0.099 0.028 88 27 80 0.5 Example 4 3.05 400.041 0.014 136 82 81 0.4 Example 5 2.65 35 0.034 0.016 172 80 86 0.3Example 6 2.83 37 0.03 0.016 138 81 81 0.3 Example 7 1.74 44 0.077 0.031153 47 81 0.4 Example 8 0.46 17 0.095 0.033 120 25 87 0.4 Example 9 0.4734 0.119 0.029 126 27 88 0.4 Example 10 1.96 25 0.048 0.016 115 75 890.3 Example 11 0.85 11 0.1 0.030 82 57 89 0.3 Example 12 1.74 44 0.0770.031 142 47 81 0.4 Example 13 2.30 58 0.083 0.030 171 59 81 0.4 Example14 1.16 59 0.097 0.022 Unmeasured 49 85 0.4 Example 15 3.40 44 0.0350.013 197 84 86 0.4 Example 16 3.20 42 0.034 0.017 150 83 82 0.4Comparative 1.30 18 0.063 0.015 180 98 18 16.8 Example 1 Comparative2.64 35 0.033 0.016 171 97 10 24.5 Example 2 Comparative 2.01 26 0.0350.016 170 100 15 27.0 Example 3 Comparative 3.20 26 0.079 0.022 193 10022 15.0 Example 4 Comparative 1.60 21 0.052 0.020 113 71 26 11.0 Example5* Reference 2.20 18 0.062 0.017 128 100 81 0.4 Example *The reactionwas carried out without rotating the reactor.

TABLE 3 Cobalt Magnesium Heat Treatment Remaining Removal RemainingRemoval Object Condition Amount/wt % Rate/% Amount/wt % Rate/% ReferenceCNF Composite Untreated  2.10 — 2.3 — Obtained in Example 4 Example 17CNF Composite 1700° C., 3 h  0.15 93 0.01 or less 99 or more Obtained inExample 4 Example 18 CNF Composite 2000° C., 1 h 0.01 or less 99 or more0.01 or less 99 or more Obtained in Example 4 Comparative CNF Composite1200° C., 3 h 1.7 19 2.1  9 Example 4 Obtained in Example 4 ComparativeCNF Composite Conducted Acid Treatment 1.2 43 0.01 or less 99 or moreExample 5 Obtained in Example 4 without Heat Treatment Reference CNFComposite Untreated 1.1 — 1.2 — Obtained in Example 7 Example 19 CNFComposite 1700° C., 3 h 0.1 91 0.01 or less 99 or more Obtained inExample 7 Example 20 CNF Composite 2000° C., 1 h 0.01 or less 99 or more0.01 or less 99 or more Obtained in Example 7 Comparative CNF Composite1200° C., 3 h 0.8 27 1   17 Example 6 Obtained in Example 7 ComparativeCNF Composite Conducted Acid Treatment 0.5 55 0.01 or less 99 or moreExample 7 Obtained in Example 7 without Heat Treatment

DESCRIPTION OF REFERENCE NUMERALS

-   -   100 reactor    -   103 rotating portion    -   104 fixing portion    -   105 gas introduction portion    -   106 stirring blade    -   107 reaction portion    -   108 gas introduction pipe    -   109 thermocouple introduction tube    -   110 thermocouple    -   111 gas exhaust pipe

What is claimed is:
 1. A method for producing a carbon nanofiber composite, comprising bringing at least one catalyst and at least one particulate carbon material into contact with at least one gas containing at least one gaseous carbon-containing compound while mechanically stirring the catalyst and the particulate carbon material in a reactor, and wherein the particulate carbon material comprises (i) 70% by volume and more of particles with a particle diameter of 1 μm or less, and (ii) a median diameter D50 by volume of 1 μm or less.
 2. The method for producing the carbon nanofiber composite according to claim 1, wherein the reactor comprises a rotary reactor.
 3. The method for producing the carbon nanofiber composite according to claim 1, wherein the carbon nanofiber composite comprises 70% by volume or more of particles with a particle diameter of 1 μm or less, and a median diameter D50 by volume of 1 μm or less.
 4. The method for producing the carbon nanofiber composite according to claim 1, wherein the particulate carbon material comprises one or more selected from graphite, carbon black, and graphene.
 5. The method for producing the carbon nanofiber composite according to claim 1, wherein the gaseous carbon-containing compound comprises carbon monoxide.
 6. The method for producing the carbon nanofiber composite according to claim 1, wherein the method comprises carrying out the reaction at a temperature ranging from 550° C. to 900° C. in the reactor.
 7. The method for producing the carbon nanofiber composite according to claim 1, wherein the catalyst comprises one or more of the following (A), (B) and (C): (A) a catalyst in which an active species mainly based on cobalt is supported on a support comprising an oxide containing magnesium; (B) a catalyst in which an active species mainly based on cobalt is supported on a support comprising an oxide containing titanium; and (C) a catalyst in which an active species mainly based on any of iron and nickel is supported on a carbon particle support.
 8. The method for producing the carbon nanofiber composite according to claim 1, wherein the method comprises allowing the gas containing the gaseous carbon-containing compound to flow such that the gaseous carbon-containing compound flows through the reactor at a flow rate of 0.1 NL/g-active species⋅minute or more.
 9. The method for producing the carbon nanofiber composite according to claim 1, wherein the gas containing the gaseous carbon-containing compound further comprises hydrogen.
 10. A method for removing a residual catalyst, comprising subjecting the carbon nanofiber composite obtained by the method according to claim 1 to a heat treatment in an inert gas atmosphere at a temperature of 1500° C. or higher and 2500° C. or lower. 